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    SubjectRe: [PATCH v2] kcsan: Fix encoding masks and regain address bit
    On Fri, Nov 06, 2020 at 10:34:56AM +0100, Marco Elver wrote:
    > The watchpoint encoding masks for size and address were off-by-one bit
    > each, with the size mask using 1 unnecessary bit and the address mask
    > missing 1 bit. However, due to the way the size is shifted into the
    > encoded watchpoint, we were effectively wasting and never using the
    > extra bit.
    >
    > For example, on x86 with PAGE_SIZE==4K, we have 1 bit for the is-write
    > bit, 14 bits for the size bits, and then 49 bits left for the address.
    > Prior to this fix we would end up with this usage:
    >
    > [ write<1> | size<14> | wasted<1> | address<48> ]
    >
    > Fix it by subtracting 1 bit from the GENMASK() end and start ranges of
    > size and address respectively. The added static_assert()s verify that
    > the masks are as expected. With the fixed version, we get the expected
    > usage:
    >
    > [ write<1> | size<14> | address<49> ]
    >
    > Functionally no change is expected, since that extra address bit is
    > insignificant for enabled architectures.
    >
    > Signed-off-by: Marco Elver <elver@google.com>

    Acked-by: Boqun Feng <boqun.feng@gmail.com>

    Regards,
    Boqun

    > ---
    > v2:
    > * Use WATCHPOINT_ADDR_BITS to avoid duplicating "BITS_PER_LONG-1 -
    > WATCHPOINT_SIZE_BITS" per Boqun's suggestion.
    > ---
    > kernel/kcsan/encoding.h | 14 ++++++--------
    > 1 file changed, 6 insertions(+), 8 deletions(-)
    >
    > diff --git a/kernel/kcsan/encoding.h b/kernel/kcsan/encoding.h
    > index 4f73db6d1407..7ee405524904 100644
    > --- a/kernel/kcsan/encoding.h
    > +++ b/kernel/kcsan/encoding.h
    > @@ -37,14 +37,12 @@
    > */
    > #define WATCHPOINT_ADDR_BITS (BITS_PER_LONG-1 - WATCHPOINT_SIZE_BITS)
    >
    > -/*
    > - * Masks to set/retrieve the encoded data.
    > - */
    > -#define WATCHPOINT_WRITE_MASK BIT(BITS_PER_LONG-1)
    > -#define WATCHPOINT_SIZE_MASK \
    > - GENMASK(BITS_PER_LONG-2, BITS_PER_LONG-2 - WATCHPOINT_SIZE_BITS)
    > -#define WATCHPOINT_ADDR_MASK \
    > - GENMASK(BITS_PER_LONG-3 - WATCHPOINT_SIZE_BITS, 0)
    > +/* Bitmasks for the encoded watchpoint access information. */
    > +#define WATCHPOINT_WRITE_MASK BIT(BITS_PER_LONG-1)
    > +#define WATCHPOINT_SIZE_MASK GENMASK(BITS_PER_LONG-2, WATCHPOINT_ADDR_BITS)
    > +#define WATCHPOINT_ADDR_MASK GENMASK(WATCHPOINT_ADDR_BITS-1, 0)
    > +static_assert(WATCHPOINT_ADDR_MASK == (1UL << WATCHPOINT_ADDR_BITS) - 1);
    > +static_assert((WATCHPOINT_WRITE_MASK ^ WATCHPOINT_SIZE_MASK ^ WATCHPOINT_ADDR_MASK) == ~0UL);
    >
    > static inline bool check_encodable(unsigned long addr, size_t size)
    > {
    > --
    > 2.29.2.222.g5d2a92d10f8-goog
    >

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